I would like to produce a report listing every bird species that I've seen (with an optional location filter) showing the most recent sighting for each. This can already be done for a location in eBird if you sort by species rather than date. It could be done using the Life List feature that already exists in Birder's Diary, because that lists one sighting for each species. If a user selects to sort by ascending taxonomic order AND by descending date, the resulting life list should be as I am requesting. But I think what I would get now is as if I requested only descending date.

The Life List report would only list the first sighting of a species (at a given location and date-range), not the most recent sighting. So I don't think that is what you want.

What is wrong with the report example I gave you in the General Discussion Forum? It gives what you are asking for, the only difference is that you get all sightings for the location, date, and observer specified. But the most recent sighting is at the top for each species. Would this request only differ from that example in that nothing else is listed except the most recent sighting?

How do you use this report?

I think eBird lists all sightings also, but you are just seeing the most recent sightings. You can do this with View & Edit Sightings window. In the setup specify your filters (location, date, observer) and sort by "Date Desc" or "Taxonomic Asc and Date Desc".